Electric-field Control Of Magnetic Properties In Epitaxial Exchange Biased Heterojunctions

As the core of spintronic devices,the exchange bias effect has been widely used in magnetic sensors and magnetic random-access memories based on spin valves and magnetic tunnel junctions.Compared with the polycrystalline exchange-biased system,the epitaxial exchange-biased system has a more controllable interface spin configuration and the competition between different magnetic anisotropies leads to an extraordinary magnetization switching processes.Using electric field to control the magnetism can not only avoid the Joule heating problem caused by the current,but also have the advantage of low energy consumption,which provides a new design idea for the magnetic control.The combination of strain-mediated magnetoelectric coupling and the exchange bias effect brings a new design idea to electrically controlled magnetic tapes.Based on the research on epitaxial FM/AFM heterojunctions,the specific results are as follows:(1)We investigated the exchange coupling in fully epitaxial Fe/FeRh/MgO(100)heterojunction.Although Fe/FeRh layers present almost perfectly symmetric hysteresis loops over the temperature region,the observed coercivity(H_c)enhancement proves the existence of exchange coupling.The direction of the exchange coupling is found to be perpendicular to the field cooling(FC)orientation,in contrast to the commonly observed direction of exchange bias for FM/AFM interfaces.The behavior of coercivity enhancement can be explained by the spin-flop model and the rotatable uncompensated FeRh moments in the interface.Almost no uncompensated pinned AFM FeRh moments results the negligible exchange bias of Fe/FeRh.(2)Epitaxial IrMn/FeGa bilayers with exchange biases along the FeGa[100]and[110]directions were prepared on MgO(001)single crystal substrates by magnetron sputtering with controlling the orientation of the external field in situ applied during growth.The effect of the exchange bias orientation on the magnetic switching processes and the magnetic switching field was studied.The measurement of the angular dependence of the ferromagnetic resonance field and the corresponding fitting to the Kittel equation show that the samples have a superposition of fourfold symmetric magneto-crystalline anisotropy₁,unidirectional magnetic exchange bias anisotropy₀0)0)0)),and uniaxial magnetic anisotropywith configuration of₀0)0)0))?[100]or₀0)0)0))?[110].The combined longitudinal and transverse magneto-optical Kerr effect measurements show that sample with₀0)0)0))?[100]exhibit square,asymmetrically shaped and one-sided two-step loops at different external magnetic field directions.In contrast,the sample with₀0)0)0))?[110]exhibit one-sided two-step and two-sided two-step loops as the magnetic field orientation changes.A model based on the domain wall nucleation and propagation was proposed with considering the different orientations of₀0)0)0)),which can nicely interpret the change of the magnetic switching route with the magnetic field orientation and fit the angular dependence of the magnetic switching fields,indicating a significant change of domain wall nucleation energy as the orientation of₀0)0)0))changes.(3)Epitaxial IrMn/FeGa exchange-biased bilayer were prepared by inserting 3nm-MgO buffer layer on PMN-PT(001)ferroelectric substrates by magnetron sputtering,and the electric control of magnetic properties was investigated.The combined longitudinal and transverse magneto-optical Kerr effect measurements show that sample with=0 k V/cm exhibit square,one-sided two-step and traditional asymmetrically shaped loops as the magnetic field orientation changes.The 90°domain wall nucleation and propagation model can well explain the magnetization switching processes and the magnetic switching field under different external magnetic field directions,and the fitting results show the initial magnetic structure is conformed to?₀0)0)0)).The magnetization switching process is significantly changed by control of electric field,as=+6 k V/cm the sample exhibits special asymmetric and square hysteresis loops.The 180°domain wall nucleation potential model is required to give results of the coercive field corresponding to the square hysteresis loop.The fitting results show that the magnetic configuration is changed to?₀0)0)0))by electric field regulation.In addition,the change of the hysteresis loop with the electric field at a specific angle was studied.The changes of K_u/M and?_90°/M under different electric fields show an asymmetric butterfly curve,which validates the realization of nonvolatile regulation with stress-mediated magnetoelectric coupling.